Currently gerrit_client retries if received 404, which does not
provide anything meaningful.
It is nature to get 404 for a non-exist branch. I am not sure why
we raise a 200 error per an empty response. I think
we could just accept 404 and return it like other GET functions, e.g.
GetAccountDetails().

BUG=1208430
TEST=local

Change-Id: I054bad99b69c54cc125141108299193f5cc092de
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3077363Reviewed-by: Michael Moss <mmoss@chromium.org>
Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Xinan Lin <linxinan@chromium.org>